Additional clones of human genomic DNA shown to have murine leukemia virus (MuLV)-like reactivity were sequenced to determine the degree of nucleotide and deduced amino acid homology with MuLV in the structural protein (gag) and the reverse transcriptase (pol) regions. Approximately 4 kilobases of human DNA were sequenced. Computer analysis of these sequences showed regions of homology which were collinear with corresponding MuLV sequences. Intervening non-homologous sequences in human DNA corresponded in number to the MuLV sequence suggesting that the human MuLV-like sequence was complete. Typical highly conserved sequences among various leukemia viruses were identified in the gag proteins, p15, p30 and p10, as well as at the junction between p10 and p01. Continuing studies on host range specificity of xenotropic MuLV focused on cloning an infectious xenotropic virus from the Hirt supernate of NZB infected mink cells. The infectious clone obtained gave positive immunofluorescence and S+L-complementation tests. In addition, the cell-free supernatant fluids from cells infected with cloned DNA were infectious.